Olefin metathesis catalysis is a powerful technology, which in recent years has received tremendous attention as a versatile method for the formation of carbon-carbon bonds and has numerous applications in organic synthesis and polymer chemistry. R. H. Grubbs, Handbook of Metathesis, Vol. 2 and 3; Wiley VCH, Weinheim, 2003. The family of olefin metathesis reactions includes, but is not limited to, ring-closing metathesis (RCM), cross metathesis (CM), ring-opening metathesis polymerization (ROMP), and acyclic diene metathesis polymerization (ADMET). The success of olefin metathesis stems from the development of several well-defined transition metal complexes, such as the Schrock molybdenum catalysts and the Grubbs ruthenium catalysts. R. H. Grubbs, Handbook of Metathesis, Vol. 1; Wiley VCH, Weinheim, 2003. The molybdenum catalysts are generally highly active but are also highly sensitive to air, moisture, and to certain functional groups present in the olefinic substrate, the reaction solvent, or impurities. The ruthenium catalysts are much more robust than the molybdenum catalysts.
The original first-generation ruthenium catalysts were primarily bisphosphine complexes of the general formula (PR3)2(X)2Ru═CHRa wherein X represents a halogen (e.g., Cl, Br, or I), R represents an alkyl, cycloalkyl, or aryl group (e.g., butyl, cyclohexyl, or phenyl), and Ra represents an alkyl, alkenyl, or aryl group (e.g., methyl, CH═CMe2, phenyl, etc.). Examples of these types of catalysts are described in U.S. Pat. Nos. 5,312,940, 5,969,170 and 6,111,121, which are incorporated herein by reference. While they catalyze a considerable number of olefin metathesis transformations, these bisphosphine complexes can exhibit lower activity than desired.
Second-generation metathesis catalysts with greatly increased activity have now been prepared by replacing one of the phosphine ligands with an N-heterocyclic carbene (NHC) ligand to give complexes of the general formula (L)(PR3)(X)2Ru═CHRa wherein L represents an NHC ligand such as 1,3-dimesitylimidazole-2-ylidene (IMES) and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (sIMES), X represents a halogen (e.g., Cl, Br, or I), R represents an alkyl, cycloalkyl, or aryl group (e.g., butyl, cyclohexyl, or phenyl), and Ra represents an alkyl, alkenyl, or aryl group (e.g., methyl, CH═CMe2, phenyl, etc.). Examples of these types of NHC ligands and catalysts are described in PCT publications WO 99/51344 and WO 00/71554, which are incorporated herein by reference. Further examples of the synthesis and reactivity of some of these active ruthenium complexes are reported by A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann, R. Mynott, F. Stelzer, and O. R. Thiel, Chem. Eur. J., 2001, 7, No. 15, 3236-3253; Blackwell H. E., O'Leary D. J., Chatterjee A. K., Washenfelder R. A., Bussmann D. A., Grubbs R. H. J. Am. Chem. Soc. 2000, 122, 58-71; Chatterjee A. K., Morgan J. P., Scholl M., Grubbs R. H. J. Am. Chem. Soc. 2000, 122, 3783-3784; Chatterjee A. K., Grubbs R. H. Angew. Chem. Int. Ed. 2002, 41, 3171-3174; Chatterjee A. K., Choi T. L., Sanders D. P., Grubbs R. H. J. Am. Chem. Soc. 2003, 125, 11360-11370. The disclosure of these articles are incorporated herein by reference.
Despite such advances in the development of olefin metathesis catalysts, the metathesis of hindered olefins remains difficult and certain metathesis reactions to form tetra-substituted olefins proceed slowly and in low to moderate yields. A need exists, therefore, for metathesis catalysts capable of efficiently and effectively carrying out metathesis reactions with hindered olefins. The ruthenium catalysts of this invention answer that need.